The main function of a utility condenser is as the name implies to condense the low energy steam exhausting from the turbine. Small diameter thin-walled condenser tubes that can number as many as 70,000 convey and contain the cooling medium (usually water) through this heat exchanger to produce the desired condensation of that steam. During operation, the waterbox side of each of the two tubesheets at the opposite ends of a condenser holds an hydraulic pressure of up to about 70 psig while the shellside of the tubesheet experiences a high vacuum as low as 1 in hga. Thus, the tube-to-tubesheet joint of a condenser must reliably withstand a differential pressure of up to 70 psig and other loads experienced during operation.
The tubesheets themselves are flexible, flat plates from ½ an inch to about 2 inches in thickness and can be of a circular, square, rectangular or a trapezoidal shape that could typically vary from say 4 ft×4 ft to 20 ft by 15 ft., depending on the size of the power plant. These tubesheet plates are of a corrosion resistant material that have been drilled to be slightly larger than the outside diameter of the tubes. The drilled hole pattern is in accordance with the designer's requirements but accommodates all the tubes and also the attachment of the tubesheet and waterbox to the shell of the condenser at its periphery.
The tubes are required to be firmly and reliably attached to their tubesheets since they must be absolutely prevented from pulling-out from their joint with the tubesheet and/or leaking any of the cooling medium into the steam side during operation. No tubes or tube joints can leak. After sticking the tubes into the condenser, the tubes are fastened at their ends into the tubesheets. Though welding is sometimes employed to make the joints, today most new and retubed condensers use tube joints that are attached to their tubesheets by a mechanical interference fit using a hand-held, manually operated tool on each joint. The tool expands the diameter of the tube into the plastic region of the material. At the same time, it expands the surrounding tubesheet hole enough so that after the tool is removed, the tubesheet material around the hole closes elastically, squeezing down on the expanded tube and producing a high contact pressure that makes the joint strong and leakproof. As has been suggested above, large condensers often require 100,000 or more of these joints to be made.
Since the 1960's, significant improvements to the leak tightness and strength of this joint have been made as the materials of the tube and tubesheets have changed away from the usual and older application of traditional copper alloys of both tube and tubesheet. Now, particularly when retubing an existing condenser, often the tube is a thin, hard material like stainless steel while the tubesheet is a relatively soft copper alloy. Historically, with copper alloy tubes, joint strength was improved using notches or grooves. Typically a quantity of two notches or grooves were machined in each tubesheet hole (@ ˜⅛″ width each). Notches or grooves had been employed by many condenser suppliers with copper bearing tubesheets to obtain better sealing and joint strength.
In the US, serrations were developed after applying an EDF (a French utility) engineer's experience from the art he presented in his paper at an EPRI condenser conference in the 1980's1. The paper showed sketches of a joint (to be used for a titanium tubed condenser) with a multiplicity (6 to 8) separate triangular shallow serrations about 1/64th inches deep and wide at the top, separated by about 1/16th inch. It demonstrated that much higher joint pullout load strength occurred with (presumably) better leak tightness. Evidently the interference fit process forced ridges to form on the tube that pressed into the tubesheet to increase its holding power. Remembering this European technology, an engineer (J M Burns now of Burns Engineering Services Inc.) with a major architect-engineering company about 6 years later specified the geometry of that serrated joint to be used by a US contractor in 1989 when retubing a US condenser that had titanium tubes and a titanium tube sheet. Tooling was developed and made by the contractor to cut those tubesheet serrations in the field; pullout tests on a sample coupon with that joint showed almost a doubling of joint strength compared to a plain joint. The joint design was subsequently cut into all of approximately 20,000 tubesheet holes of that condenser. That engineer wrote a paper about the experience several years later to inform the industry2. Though it was judged to be adequate, the metal-to-metal tube contact with harder metals did not extend for the full depth into the tubesheet serrations or grooves and so some potential strength was likely lost. It was also found that all shards of metal needed to be deburred from the process, and brushed out from the grooves formed within each tubesheet hole before being able to stick the tubes into the condenser through those close clearance holes. In addition, scratches often occur on the outer tube surfaces and the metal serration points can sometimes be knocked off as the tubing is inserted through the hole. Not only are these scratches a potential site for tube failure, the small metal particles/chips that can get knocked off in the serrations interfere with making a fully reliable joint when swaged. 1!983 EPRI Technical Conference2J M Burns, et al, “New Approach to One-For-One Retubing of Titanium Condenser Improves Reliability For Continued Unit Operation”, EPRI Technical Conference, 1993.
Since then, when specified by the utility owner or his representative in the US, the same serrated joint enhancement as described above has been made and used in the field by most contractors without much variation. It typically consists of a quantity of 6-8 machined narrow cuts of depth 1/64″ with each serration tip spaced approximately 1/16″ apart. Note however the standard size or quantity is not a written industry standard.
Nonetheless, there were aspects of this serrated joint (as suggested above) that can be improved to make it even stronger and to make it more cost effective to install than the current practice. Cutting in the grooves takes time, labor and tooling. To put it into context, each of up to 100,000 tube holes must have a cutting tool properly positioned and then the tubesheet hole must be carefully serrated. After the cutting, each hole must be deburred and brushed-out as necessary; then the hole must be inspected and possibly rebrushed before the tube can be pushed into the condenser and finally swaged into place. If it optimistically takes only 1 minute for the entire serrating process of each tubesheet hole of a 50,000 tube unit, including the hole brushing, that is a total of over 1,600 man hours. The enormous quantity of holes to be serrated for the joint also usually indicates some holes will be missed, not properly brushed-out or inspected before the tubes are pushed. This can result in a bad tube installation and/or a poor tube-to-tubesheet joint.